1. Field of the Invention
The present invention relates to a displacement measuring method for measuring a displacement of a cantilever accurately, and more particularly, to a scanning probe microscope that can observe, by using the method, a surface shape or physical property information of a sample by scanning a surface of the sample with a probe.
2. Description of the Related Art
The scanning probe microscope (hereinafter, referred to as SPM) is a microscope for measuring a surface shape or physical property information of a sample, and there are proposed various measuring modes of the SPM. For instance, there are a scanning tunneling microscope (hereinafter, referred to as an STM) that maintains a tunnel current flowing between the probe and the sample to be constant so as to obtain the surface shape, and an atomic force microscope (hereinafter, referred to as AFM) that detects an atomic force between the probe and the sample so as to obtain the surface shape. Among the AFMs, there are a contact mode in which bending of the cantilever is maintained to be constant so as to obtain the surface shape, and a dynamic force mode (hereinafter, referred to as DFM mode) in which the cantilever is vibrated and its amplitude is maintained to be constant so as to obtain the surface shape. Usually, the probe and the sample are brought into contact with each other constantly in the contact mode and intermittently in the DFM mode.
In the SPM, it is possible to perform physical property measurement in which the sample surface shape and the physical property information of the sample are obtained simultaneously. In particular, through the use of a conductive cantilever having a metal coated probe, it is possible to measure an electrical property of the sample surface. For instance, there are a Kelvin probe force microscopy (hereinafter, referred to as KFM) for observing a surface potential of the sample, an electric force microscopy (hereinafter, referred to as EFM) for measuring an electric force, and a magnetic force microscopy (hereinafter, referred to as MFM) that can observe a magnetic characteristic. In each measuring method, the probe is kept not in contact with the sample so that physical interactive action between the probe and the sample surface is detected as a displacement of the cantilever (for example, see Takeharu Yamaoka, Material Technology, Vol. 23, No. 4 (2005) 211).
It is possible to measure various types of physical property information of the sample surface in a state in which the probe is brought into contact with the sample, namely in a contact mode state. For instance, there is a scanning nonlinear dielectric microscopy (hereinafter, referred to as SNDM) that can measure a capacitance change of the sample surface. The SNDM measures a capacitance change of the sample when an AC voltage is applied between the conductive cantilever and the sample. As a method of measuring the capacitance change, a conductive cantilever is attached to an LC resonator, and a change of capacitance just below the probe is converted into a change of resonance frequency of the LC resonator. Further, the change of resonance frequency is converted into a change of voltage by an FM demodulator or the like, to thereby detect the capacitance change as the change of voltage (for example, see Japanese Patent Application Laid-open No. Hei 8-75806).
On the other hand, as a method of detecting the displacement of the cantilever, an optical cantilever method is usually used. As the optical cantilever method, there is a method in which light such as a laser beam is applied to a back surface of the cantilever, and a position of reflected light thereof is detected so that bending (displacement) of the cantilever can be detected. As a method of not using light, there is a piezoresistance type self-detecting cantilever using piezoresistance. In this type, a piezoresistive strain sensor is mounted on the cantilever so that bending of the cantilever can be detected as a resistance of the piezoresistor (for example, see Japanese Patent Application Laid-open No. 2007-532923).
In the AFM, it is usual to use the optical cantilever method for detecting a displacement of the cantilever. However, because light is applied to the sample too, the physical property measurement may be affected by the light. In particular, when a potential or current of the sample surface is measured, the sample itself may change its physical property value by photoexcitation or the like so that accurate physical property information cannot be measured. In addition, for the same reason, it may be difficult to irradiate the sample with light so as to measure a change of physical property information of the sample between presence and absence of the light.
On the other hand, the method of using the piezoresistance type self-detecting cantilever does not use light for measuring a displacement of the cantilever, and hence the method does not have the above-mentioned influence. However, there are various restrictions when the electrical property is measured with the KFM or the like. For instance, the cantilever to be used should have a piezoresistor so that a complicated structure and a special manufacturing process are necessary. As a matter of course, a usual cantilever available in the market cannot be used. In addition, in the electrical property measurement, a conductive cantilever having a metal coated probe is usually used, but the piezoresistance type self-detecting cantilever having a metal coated probe requires a more complicated structure and manufacturing process, and hence there is difficulty in manufacturing the cantilever itself. In addition, in the method such as the KFM or the EFM in which an alternating electric field is applied between the sample and the probe, the applied alternating electric field is also applied to the piezoresistance sensor. As a result, the displacement current generated in the piezoresistor may be mixed into a displacement signal of the lever as noise, and hence accurate measurement cannot be performed. Concerning this point, there is reported a measurement example of the KFM utilizing a phase difference between an alternating electric field affecting the piezoresistance sensor and an alternating electric field affecting between the probe and the sample, but a structure of the apparatus is complicated, and the resolution is low (see M.Takahashi, T. Igarashi, T. Ujihara and T. Takahashi: “Photovoltage Mapping on Polycrystalline Silicon Solar Cells by Kelvin Prove Force Microscopy with Piezoresistive Cantilever”, Jpn. J. Appl. Phys., 46, 548 (2007)).